In time-of-flight (TOF) mass spectrometry, flight times of ions are measured to determine mass-to-charge (m/z) ratios. As is well known, the time of flight of an ion is proportional to the square root of its mass to charge ratio. The recorded time of detection is linked to the m/z ratio by a calibration function. The ambient temperature of a mass spectrometer can vary by more than 10 degrees Celsius during use, which leads to thermal expansion of the mechanical parts and thermally induced drift of the electronic components (voltage supplies). Variations in temperature of the TOF-MS lead to changes in the measured time of flight of ions of a given species. For a given calibration function, this leads to a negative effect on the accuracy of the calculated mass to charge ratio of that ion species, when conditions change after the initial calibration function has been determined.
Several approaches have been taken in the past to minimize these effects. In U.S. Pat. No. 6,049,077, a mix of appropriate materials is used in order to try to maintain a constant flight path as the temperature changes. A different solution has been proposed in U.S. Pat. No. 6,465,777, where the temperature of critical mechanical and electronic components is kept constant by the use of an air flow mechanism.
In U.S. Pat. No. 6,700,118, several sensors are employed to obtain temperature and strain measurements from the instrument. The measured parameters are then used in conjunction with a mathematical model to provide adjusted mass spectra.
Yet another approach is presented in US-A-2008/0087810. In this case, the length of the flight path is determined at a reference temperature of the assembly. Predetermined thermal expansion correction factors for the flight path assembly are then employed for correction. The correction is carried out by appropriately controlling another component of the TOF MS, such as the voltage applied to a power supply system, or a signal to control clock frequencies.
In U.S. Pat. No. 6,797,947, an internal calibration source is used to achieve high mass accuracy. So called lock mass ions with exactly know masses are mixed with analyte ions prior to mass analysis. The recorded mass spectrum contains peaks of known lock mass ions and analyte ions whose m/z ratio can be determined with high accuracy.